refactor: Remove colon from quote format

Format changed from @[User]: »text« to @[User] »text«
Guillemets alone are sufficient to separate the quote.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/static/js/app.js

@@ -782,7 +782,7 @@ function quoteTo(username, content) {
         quotedText = truncated + '...';
     }
 
-    input.value = `@[${username}]: »${quotedText}« `;
+    input.value = `@[${username}] »${quotedText}« `;
     updateCharCounter();
     input.focus();
 }